1. Field of the Invention
This invention relates to magnetrons of the type wherein choke coils for suppressing leakage of unwanted high frequency components can be prevented from overheating.
2. Description of the Prior Art
In magnetrons widely used for, for example, electronic ovens, leakage of unwanted high frequency components such as harmonics interferes with, for example, TV reception and is sometimes damaging to living creatures. To suppress such a leakage, it is common practice to use a magnetron of the type wherein a filter circuit comprised of a series connection of choke coils and a through-hole type capacitor is connected in a power feeder line of a cathode filament and housed by a hermetic shield case.
Exemplified in sectional form in FIG. 1 is a magnetron provided with the filter circuit which comprises an anode structure having a cylindrical anode electrode 1 and a plurality of vanes 2 mounted to the inner wall of the anode electrode. An interaction space 3 is formed between the plurality of vanes 2 and a cathode electrode 4 disposed concentrically with the anode electrode. Magnetic pole pieces 61 and 62 mounted to opposite ends of the anode electrode 1 serve to guide magnetic flux fed from a permanent magnet 63 by way of a yoke 6 to the interaction space 3.
An output terminal 5 opposes one end of the anode structure through a metal member 51 and an insulating member 52. Cathode input terminals 7 and 7' extend from a pair of lower and upper end caps 53' and 53 for supporting the cathode electrode 4 in the opposite direction to the output terminal 5 and pass through a metal tube 71 and a cathode stem insulating member 72. Coils 15 and 15' are connected in series with a capacitor 16 to form a filter circuit 8 to which lead wires 9 and 9' are connected which extend exteriorly of a shield case 10 adapted to house the filter circuit. The shield case 10 has a bottom 10a sealingly mated with an opening of the yoke 6 and an upper opening 10b covered with a lid 11 firmly secured to the case 10 by the aid of blind rivets 12. The coils 15 and 15' are wound about high frequency absorbers 17 and 17' and fixed thereto by an adhesive 18.
FIG. 2 shows the interior of the shield case of a conventional magnetron with the lid removed. In the figure the same elements as those in FIG. 1 are designated by the same reference numerals. Shown therein are cathode input terminals 7 and 7', choke coils 15 and 15', through-hole type capacitor 16, shield case 10, rod shaped ferrite cores 17 and 17' serving as high frequency absorbers, and coils 15 and 15' having coil windings 22 and 22'.
In operation of the magnetron, high frequency energy is generated in the interaction space 3 as defined by the vanes 2 and reaches the output terminal 5 for propagation to an electronic oven via a wave guide tube not shown. However, the high frequency partly reaches the cathode electrode 4 and leaks, via the cathode input terminals 7 and 7', to the coils 15 and 15' and high frequency absorbers 17 and 17' constituting the filter circuit 8. Such a leakage is especially aggravated when the electronic oven is operated without an object to be heated. As well known in the art, the size of the choke coil for a given inductance can be reduced by inserting a high permeability core in the coil. A ferrite core is desirably used with the magnetron choke coil because of its versatility in performance. Specifically, a large high frequency power loss is caused in ferrite placed in a high frequency electromagnetic field and as a result the intensity of the electromagnetic field is greatly attenuated. In other words, the ferrite core inserted in the choke coil effectively acts as a high frequency absorber which can effectively prevent the leakage of high frequency. But the ferrite core is in turn heated to a high temperature with the result that a high voltage develops across adjacent coil windings of the coil wound about the ferrite core. Consequently, there were encountered in the conventional magnetron problems that the covers of coil windings of the choke coil close to the cathode input terminal are burnt and the ferrite core is deteriorated or cracked owing to heat at high temperatures. Disclosed in Japanese Utility Model Kokai (Laid-Open) No. 58944/77 (laid open Apr. 28, 1977) assigned to the same assignee as the present application is an expedient to solve this problem, according to which a choke coil has in part air-core coil windings close to the cathode input terminal. More particularly, the choke coil according to this expedient has, as shown in FIG. 2, 12 turns in total and 4 turns close to the cathode input terminal 7 or 7' are deprived of the ferrite core 17 or 17'. This expedient alleviates the problem but is not so effective as to fulfil its intended effect for a variety of magnetrons of different specifications. An essentially similar expedient to the above has been proposed in Japanese Patent Kokai (Laid-Open) No. 37348/78 (laid open Apr. 6, 1978). What is different from the former proposal resides in that two partial air-core coil windings have different inductances. Specifically, one air-core coil winding of the coil 15 connected to one (7) of the two cathode input terminals 7 and 7' which is coupled with the center lead for cathode support has a smaller number of turns than the other air-core coil winding of the coil 15'. With this latter expedient, however, the inventors have experienced certain experimental results which are inferior to those obtained with the former. Japanese Utility Model Kokai (Laid-Open) No. 101962/79 discloses another proposal in which the ferrite core is applied only to a central coil winding of the choke coil, forming air-core coil windings at opposite ends. However, one air-core coil winding of this expedient close to the cathode input terminal simulating the aforementioned Japanese Utility Model Kokai (Laid-Open) No. 58944/77 is not always fully effective and the other air-core coil winding close to the capacitor, which is not essentially subject to high temperature rise, does not play an important role. Nevertheless, the total length of the choke coil according to this proposal is sometimes required to be prolonged in order to prevent high frequency leakage to the same extent as the present invention. Japanese Utility Model Kokai (Laid-Open) No. 42458/79 (laid open Mar. 22, 1979) discloses still another proposal in which at least one of the paired choke coils has a prolonged end wiring, making a detour merging into the cathode input terminal. However, the detouring end wiring tends to contact the shield case and it is practically impossible to make so long a detour of the end wiring as to attain sufficient effects without enlarging the shield case in view of the electrical insulation because the end wiring to the cathode electrode is at a high negative potential relative to the shield case at earth potential.